1. Field of the Invention
The present invention relates to a radio communication system which comprises a base station and a plurality of personal radio apparatuses (terminal stations).
2. Description of the Prior Art
Radio communications are suitable for establishing communication networks without communication lines and for establishing communication with mobile units, and known examples of a radio communication system include personal communication systems which allow simple and easy establishment of communication networks in areas without telephone networks which imitate telephone networks.
As shown in, for example, FIG. 4, personal radio systems consist only of terminal stations (personal radio apparatuses) 1, with no base station, and are purpose-built systems for distributed processing, wherein each terminal station has a non-directional antenna la and is operative separately; a characteristic aspect thereof is in that any subscriber may enter the radio communication network upon purchase of personal radio equipment, and thus the system is extremely simple and easy to construct and operate. As many as 1,500,000 personal radio apparatuses are presently used in Japan.
Personal radio systems are MCA (Multi Carrier Access) radio systems which allow joint use by the subscribers of eighty radio frequency channels confined within a 900 MHz band, wherein searching for unallocated channels and setting of channels are conducted with a controlling channel at each terminal station, and communication is performed between the subscribers with matching identification numbers in a press-to-talk method on an identical radio carrier frequency.
Two thirds of the population in the world live without telephone, and a variety of social problems are caused because of this lack of communication means. The aforementioned personal radio system is the best way of establishing simple and easy communication networks in such areas without telephones. Existing personal radio systems, however, are established for the purpose of communication within extremely confined areas, and this presents a problem in that the systems cannot be directly used for networks extended over relatively wide areas.
More specifically, the existing personal radio systems have no base station, and horizontal non-directional antennas are used at the respective terminal stations. Accordingly, the distance which a carrier signal may travel is restricted due to a low antenna gain. For example, in FIG. 4, the terminal station A may communicate with B and C, but fails to communicate with the terminal station E at a considerable distance therefrom.
For this reason, in order to allow establishment of a communication network over a relatively broad area, it becomes necessary to settle a base station as the core of the target area and to perform amplifying relay. However, as mentioned above, since an identical frequency is employed for both transmitting and receiving, usual ways for amplifying relay cannot be adopted. As a system for amplification at a relay station in such cases, there has hitherto been known, for example, the amplifying relay system shown in FIG. 5, which is a candidate system for communication over a wide area.
The amplifying relay system shown in FIG. 5 is disclosed in Japanese Laid-Open Patent Application SHO No. 63-79429 (published Apr. 9, 1988), which comprises a receiving antenna 41, a directional coupler 42, a detection circuit 43, a variable attenuator 44, a frequency converter 45, an oscillator 46, an amplifier 47, a control circuit 48 and a transmitting antenna 49. This amplifying relay system is designed so as to shift the frequency of the received signal slightly to generate the frequency of the transmitted signal at the frequency converter 45, to detect the coefficient of coupling due to a sneak signal from the transmitting antenna 49 to the receiving antenna 41 and to control the magnitude of attenuation by the variable attenuator 44 through the control circuit 48 in order to maintain stability of the loop.
With this conventional amplifying relay system which involves frequency conversion to distinguish between the received signal and the transmitted signal, however, since a rather large difference must exist between the frequencies in order to ensure reliable operation of the detection circuit 43, the system has the problem of difficult reception by personal radio equipment (a terminal station) which utilizes an identical frequency for transmitting and receiving signals.